1. Field of the Invention
The present invention relates to a cam driving device and a processing method, and particularly to a cam driving device and a processing method that are suitably used for a processing apparatus for processing the inner peripheral surface of a non-circular bore.
2. Description of the Related Art
It has been general in a manufacturing process of vehicles that a bore of a cylinder block of an engine is formed by a cutting work and then a cylinder head, a crank case, etc. are installed in the cylinder block. Furthermore, a piston is also mounted in the bore. When an actual engine is driven, fuel in the engine is combusted, and the piston is heated by combustion gas and expands. At this time, the shape of the piston approaches to a perfectly circular shape in section, and thus the piston has a substantially perfectly circular cross-section. Therefore, when the piston is installed in the bore of the cylinder block, the piston has an oval shape in which the major axis is slightly longer than the minor axis. In some cases, it is desired that the cross-sectional shape of the bore is a desired elliptical shape having an outward fixed clearance with respect to the outline of the cross-section of the piston. Therefore, it is desired to perform the cutting work so that the cross-sectional shape of the bore approaches to such a state that a fixed range of clearance is formed between the cross-section of the bore and the piston, and it is also desired that the bore has a substantially perfectly circular cross-section when the engine is driven after the engine is assembled.
It has been hitherto difficult to process bores in a non-circular shape containing a desired elliptic shape at high speed, and thus bores have been hitherto processed in a substantially perfectly circular shape. Even when the bore of a cylinder block is processed to have a desired elliptical cross-section, the shape of the bore is deformed when a cylinder head, a crank case, etc. are assembled to the cylinder block. When the bore is deformed as described above, it increases the sliding resistance between the bore and the piston when the engine is used, and thus there is a risk that the engine cannot exercise desired performance.
Therefore, in the process of processing the bore of the cylinder block, the bore processing is executed while a dummy head imitating the cylinder head is attached, and when the bore processing is finished, the dummy head is detached. However, when the attaching and detaching work of the dummy head, etc. is executed every time the processing of the bore of the cylinder block is executed, the productivity is greatly reduced.
In order to solve this problem, the following method has been proposed (see JP-A-2007-313619). That is, a dummy head is mounted in the cylinder block, and a bore is processed to have a non-circular shape containing a desired elliptic shape in cross-section or an ideal circular shape containing a substantially perfectly circular shape in cross-section. Subsequently, the dummy head is detached from the cylinder block. At this time, stress caused by the mounting of the dummy head is released, and thus the shape of the bore is deformed and becomes a non-circular shape in cross-section. The overall shape of the bore having the non-circular cross-section is measured to generate NC data. Specifically, measurement points are set at predetermined intervals along the axial line of the bore which has been deformed to have a non-circular cross-section due to the detachment of the dummy head, and the cross-sectional shape of the bore at each measurement point is measured, thereby generating the NC data described above.
Thereafter, boring processing is executed on a non-processed cylinder block on the basis of the generated NC data without mounting any dummy head to form a non-circular bore. According to this method, even in a case where the bore is processed without securing the dummy head to the cylinder block, the bore has a three-dimensional shape having a desired ideal elliptical cross-section when the cylinder is mounted.
However, the deformation amount of the bore when the dummy head is detached from the cylinder block is not necessarily uniform, and thus the cross-sectional shapes of the bore at the respective measurement points are different from one another. Accordingly, in order to form a non-circular bore in the cylinder block, it is necessary to execute the cutting work corresponding to the cross-sectional shape at each of the measurement points along the axial line. The cutting work is executed while a cutting tool is projected to the inner peripheral surface of the bore. In order to process the bore so that the bore have different cross-sectional shapes at the respective measurement points, the projection amount of the cutting tool at each measurement point is required to be accurately set. When the control of the projection amount is not accurate, the cross-section of the bore does not have a desired ideal elliptical shape when the cylinder head is assembled to the cylinder block, and this causes increase of the sliding resistance between the bore and the piston when the engine is used.